1. Field of the Invention
The present invention relates to a power unit for driving a driven member, and more particularly to a power unit having an internal combustion engine and rotary motors as power sources.
2. Description of the Related Art
Conventionally, as a power unit of this kind, one disclosed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2002-281607 is known. This power unit is for driving drive wheels of a vehicle, and is provided with an internal combustion engine, a first rotary motor, and a second rotary motor, which are power sources, a first planetary gear unit and a second planetary gear unit, which are of a general type. A carrier of the first planetary gear unit and a sun gear of the second planetary gear unit are directly connected to each other without using a gear or a transmission and are directly connected to the engine. A ring gear of the first planetary gear unit and a carrier of the second planetary gear unit are directly connected to each other and are connected to the drive wheels. Further, a sun gear of the first planetary gear unit and a ring gear of the second planetary gear unit are directly connected to the first rotary motor and the second rotary motor, respectively.
In the power unit configured as above, the drive wheels are driven using power from the engine and the first and second rotary motors according to traveling conditions of the vehicle. Further, power generation is performed by the first rotary motor using part of power from the engine, to supply the generated electric power to the second rotary motor, and the rotational speeds of the first and second rotary motors are controlled to thereby transmit power from the engine (prime mover) to the drive wheels while changing the rotational speed of the engine.
In the above-described conventional power unit, depending on traveling conditions of the vehicle and operating state of the engine, torque from the first and second rotary motors sometimes becomes too large, or the rotational speeds of the first and second rotary motors sometimes become very high. However, in the conventional power unit, the first and second rotary motors are directly connected to the sun gear of the first planetary gear unit and the ring gear of the second planetary gear unit, respectively, and therefore, to cope with the former case, it is required to use large-sized rotary motors as the first and second rotary motors. Further, for the same reason, in the latter case in which the rotational speeds of the first and second rotary motors become too high, dragging losses are caused due to dragging of the first and second rotary motors. This results in the degraded efficiency of the whole power unit.
Furthermore, in the conventional power unit, in changing the rotational speed of the engine as described above, particularly during a high-speed operation of the vehicle in which the rotational speed of the drive wheels is higher than that of the engine, the rotational speed of the second rotary motor becomes very high due to the above-described connection relationship, which sometimes causes dragging losses. To avoid this problem, it is only required to increase the rotational speed of the engine to decrease the rotational difference between the rotational speed of the drive wheels and that of the engine. In this case, however, the best fuel economy of the engine cannot be obtained, and the efficiency of the whole power unit is degraded after all.